1. Field of the Invention
The present invention relates to an optical data recording/reproducing apparatus and, more particularly, to an optical data recording/reproducing apparatus such as an addition type or an exclusive playback type optical disk apparatus or an erasable or rewritable photomagnetic disk apparatus.
2. Description of the Prior Art
An optical data recording/reproducing apparatus of the type described is disclosed, for example, in Terada "Essentials of Optical Pickup System Design", Japan Technology Center, [6], Oct. 31, 1984, pp. 151, 152 and 161. As taught in this document, an optical data recording/reproducing apparatus, such as an optical disk apparatus, has an optical pickup which emits a light beam onto a recording layer provided on an optical disk which is a specific form of an optical recording medium. Receiving a reflection from the recording layer, the pickup detects a focus error and a tracking or tracing error to output signals for focusing servo control and tracking servo control. A servo error signal generating means generates a focus error signal and a tracking error signal in response to the output signals of the pickup. A drive means produces a drive current for focusing and a drive current for tracking on the basis of the focus error signal and tracking error signal. An actuator moves the pickup in a focusing direction and a tracking direction by the drive currents.
An optical disk apparatus having the above construction rotates an optical disk by a spindle motor or similar drive source and reads data out of the disk while effecting feedback control (servo). Specifically, the drive means moves the pickup in the focusing direction and tracking direction via the actuator in response to the focus error signal and tracking error signal fed thereto from the servo error signal generating means, thereby correcting the focus error and tracking error. An optical disk for use with this type of apparatus has great capacity and durability due to optical recording and reproduction of data and is extensively used for various purposes such as document filing.
However, the problem with the conventional optical disk apparatus, for example, is that it is not operable with different kinds of optical disks, i.e, the kind of usable disks is limited on a manufacturer basis or on a model basis. More specifically, optical disks in general share substantially the same characteristics regarding reflectance and recording sensititivity . This stems from the fact that the quality and quantity of a material for forming a recording layer by sputtering or vacuum evaporation can be controlled relatively easily. However, the physical configuration of tracks or channels provided on an optical dick such as the depth and shape depends on the kind of the disk, i.e., the type of the disk with which a particular optical disk apparatus is operable.
For the reasons described above, when different kinds of opticals disks are used with a single optical disk apparatus, the amplitude of a tracking error signal produced by the optical pickup noticeably differs from one disk to another. Assume that the apparatus has the gain of tracking servo thereof, i.e., the tracking gain, matched to a particular kind of optical disks. Then, should the amplitude of the tracking error signal be great, the tracking gain would be excessively great and cause the tracking servo loop to oscillate. Conversely, should the amplitude of the tracking error signal be small, the tracking gain would be inadequate and cause the tracking error to increase.
Another drawback particular to the playback of different kinds of optical disks by a single apparatus is that the offset of tracking servo, i.e., the tracking offset produced by the pickup, greatly differs from one disk to another. Specifically, an optical disk apparatus whose tracking offset is matched to a certain kind of optical disk cannot play back the other kinds of disks without being accompanied by different tracking offsets. The change in the tracking offset would cause the tracking position to deviate and thereby degrade the C/N (signal-to-noise) ratio of a reproduced signal. Moreover, on the deviation of the tracking offset, the optical pickup is apt to move beyond the expected tracking control range, preventing normal tracking from being performed. Then, the pickup would uncontrollably run toward the innermost edge or the outermost edge of the disk.
As discussed above, the conventional optical disk apparatus lacks interchangeability as to optical disks and, therefore, the portability (replaceability) which is one of outstanding features thereof cannot be made most of. While various attempts have been made to reduce the access time and cost to implement a high performance optical disk apparatus, the interchangeability of optical disks is the pressing question.
Considering the long life (e.g. more than ten years) and the large capacity of optical disks, it is preferable that the optical disk apparatus also have a long service life. Optical disks having greater capacities, i.e., higher bit densities or higher track densities, are available today. This kind of disks need a servosystem capable of following the tracks accurately with a focus error of less than .+-.1 micron. Therefore, when the apparatus is used over a long period of time, it is likely that the focus deviates due to the deflection of a base which supports the pickup or the deterioration of various elements incorporated in the pickup. The deviation of focus prevents a light beam from being focused on a disk and thereby degrades or practically makes the recording and playback function unattainable.
Customarily, an optical disk apparatus has the focus offset thereof adjusted by the manufacturer before shipment and lacks a user-oriented implementation for readjustment. Hence, when the above-mentioned base is deflected or the various components of the pickup are deteriorated due to aging, it is difficult for the user to readjust the focus. The deviation of focus due to aging lowers the C/N ratio of sensed signals and thereby reduces the reading rate or, in the worst case, prevents track addresses and sector addresses from being read out of an optical disk while disabling the recoding function as well.
The problems described above are brought about not only with optical disk apparatuses but also photomagnetic disk apparatuses. Optical disks, photomagnetic disk, optical cards and other similar recording media will be collectively referred to as optical recording media hereinafter.